The binders used for manufacturing sand molds for foundries currently used have a number of drawbacks regarding the environment and their associated capital and operational costs which require long manufacturing times and also cause problems in the elimination of sand from the metal castings after solidification in the molds.
The binders currently used are based typically on phenol resins and polyisocyanates which do not require heat for curing or hardening, but instead need catalysts and chemicals, for example amines (such as triethylamine) or carbon dioxide. Handling these materials however cause health and safety problems, because the heat of the cast metal in the molds produce toxic vapors and gases. There exist also other binders containing furans which are cured by reaction with sulfur dioxide similarly causing environmental problems.
There is a need in the aluminum and other light metal foundry industry (especially for automotive and aviation aluminum castings) for a sand binder which overcomes the disadvantages of the current binders; while also being of low material cost, not emitting toxic gases, and being utilized at low operational cost (with regard to reducing the energy used for its curing and for its elimination after solidification of the cast products).
There are several proposals already in the market for binders based on sodium silicate and a carbohydrate, which are soluble in water and which do not show the environmental problems of the phenol and urethane resins. However, these prior silicate based binders unsatisfactorily lack or are insufficient in regard to other important desirable properties. For example, the prepared sand incorporating such binders lacks a suitable fluidity for the sand to flow effectively when blown by air into the mold forms (where the sand takes the predetermined geometric shape). Additionally, the binding strength of the resulting molds and/or cores tends to be insufficient in the mechanical strength needed to withstand the handling and pressure of the liquid metal in order to maintain their precise dimensions. Also such prior sand molds tend not to be easily destroyed after solidification of the molten metal (as needed to allow for efficient sand reclaiming and recycling).
The above-mentioned desirable qualities missing from such prior art is provided by the binder of the present invention, which comprises the following ingredients: (1) an alkali metal silicate (e.g. NaSiO2); (2) an alkali metal hydroxide (e.g. NaOH); (3) a starch (e.g. corn &/or tapioca starch); (4) an alkyl silicate (e.g. tetraethyl silicate); and (5) water.
Some of the publications of the prior art found in connection with the present invention are the following:
U.S. Pat. No. Re 35334, which is a reissue of U.S. Pat. No. 5,089,186 granted to Moore et al, describes a method that eliminates the sand core of a molded product that can be formed from plastic or metal, where the sand is bound by a binder composed of an alkali silicate and a carbohydrate, which can be a saccharin or a starch. The binder used is cured by heat. The core and the product are exposed to water, preferably hot water in a bath or steam for its rapid disintegration and elimination from the molded product.
U.S. Pat. No. 5,248,552 describes a sand core bound with sodium silicate and a carbohydrate which can be a starch. This patent is a divisional application of U.S. Pat. No. 5,089,186 described above, and claims a sand core, whereas the prior patent claims the method to eliminate the sand core from the molded metallic or plastic product.
U.S. Pat. No. 4,070,196 describes a foundry mold and core composition whose disintegration is favored by adding a hydrolyzed starch with the sodium silicate. The hydrolyzed starch has a dextrose equivalent of less than 5.
U.S. Pat. No. 4,226,277 describes a sodium silicate binder that also comprises additives such as alumina, borax, clays, kaolin, bentonite, iron oxide and graphite. It also suggests adding sugars such as dextrose, glucose, and other polysaccharides. This patent does not suggest adding an alkyl silicate nor starch mixtures in the binder composition. Although it mentions the use of hot air to cure the sand probes, the binder composition is very different.
U.S. Pat. No. 4,329,177 describes the composition of an aqueous solution useful to manufacture sand cores for foundries that is obtained by mixing an alkali silicate and a water soluble carbohydrate (monosaccharides, polysaccharides and their derivates). It also includes urea. This binder is cured with CO2 which has disadvantages in respect to the curing process that only required heat. This patent also does not suggest the utilization of an alkyl silicate in the binder composition.
U.S. Pat. No. 4,763,720 describes the manufacturing of a sand mold that is easily disintegrated, in which the sand is bound by two binders: an inorganic one that is sodium silicate, and an organic one that is a starch. The starch and water are mixed first and are then added to the sand along with the silicate. The core is preferably cured by microwaves. This patent however does not mention or suggest the use of an alkyl silicate, nor does it suggest the use of hot air to harden the binder used in the processes and equipment currently used in the industry.
None of the patents mentioned above suggest adding an alkyl silicate to the binder composition, nor suggest the use of hot air in a cold box process with the tooling and equipment currently used with phenol-urethane binders to harden the core or mold formed.
The patents mentioned above mention only two of the basic ingredients of the present invention, and no prior art has been found that shows the use of an alkyl silicate in combination with sodium silicate and sodium hydroxide, and a binder as in the present invention.
The binder here claimed gives the resulting sand and binder mixture qualities of fluidity, mechanical resistance and ease of sand elimination after the metal solidification, such as to provide these several significant advantages over the seemingly similar processes currently used to manufacture molds and cores.